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The Cost of Using Electrical Appliances (Higher)
Transfer of Energy | Efficiency | Measuring Energy and Calculating Cost
 
Transfer of Energy

Much of the energy transferred in homes and industry is electrical energy. This is because it is easily transferred as:
  • heat (thermal energy)
  • light
  • sound
  • movement (kinetic energy)

For example, a hairdryer transfers electrical energy as heat and movement (kinetic). Some energy is wasted as sound.

 

Efficiency

The more useful energy an appliance transfers, the more efficient it is.

Efficiency can be calculated using the following equation.

 

Efficiency =
useful energy transferred by device
total energy supplied to device
  

For example, if the hairdryer transferred 1000 J of electrical energy into 350 J of heat, 150 J of movement energy and 500 J of sound, its efficiency would be calculated as follows.

 

Efficiency

=

 

=

 

=
350 + 150
1000

500
1000
 
 
0.5

The amount of energy that an appliance transfers depends on:

  • how long the appliance is switched on for
  • how fast the appliance transfers energy (its power)

 

 12.4_electrical_efficiency

Measuring Energy and Calculating Cost

Energy is measured in joules (J), but this is a very small amount of energy. Instead electricity supply companies use a much bigger unit called the kilowatt-hour (kWh), or simply a Unit.

The power of an appliance is measured in watts (W) or kilowatts (kW)

1 kW = 1000 W

The number of kilowatts-hours (Units) of energy transferred to an appliance is calculated using this formula.

 

Energy transferred
=
power

x

time

  
 

(kilowatt-hour, kWh)

  

(kilowatt, kW)

  

(hour, h)

  


The cost of this energy can be calculated using this formula.

  Total cost
=
number of Units
 x cost per Unit  

For example, if a washing machine has a power rating of 3 kW and is used for 4 hours per week what is the total cost if one Unit of electrical energy costs 8 pence?

  Number of Units used
=
 power x time
   
=
 3 x 4
   
=
 12 Units    
           
  Total cost = number of Units x cost per Unit  
    =
12
 x   8p
    =
96p
      

The total amount of electrical energy transferred by a device can be calculated using:

 

energy transferred
=
power
x
 time  
 
(joule, J)
  

(watt, W)

   (second, s)  

So, to calculate the amount of energy transferred by the same washing machine switched on for 4 hours:

  power in watts
=
 3000          
  time in seconds
=
 4 x 60 x   60    
   
=
 14,400          
               
  energy transferred
=
 power x   time    
   
=
 3000 x   14400    
   
=
 43,200,000 J